The present invention relates to a collector for a liquid droplet radiator. More particularly, it relates to a collector for a liquid droplet radiator and which includes a housing and a pump.
The rejection of heat in space is a critical aspect of virtually all proposed space-borne installations, from solar power satellites to low temperature materials processing laboratories. On purely thermodynamic grounds it is desirable to reject heat at as low a temperature as possible, for instance, to maximize efficiency in a power cycle or to minimize the work required to drive a heat pump, and this implies a large radiator area and mass. This obviously conflicts with the basic requirement that any spaced-based installation have minimal mass. The balance between device performance and radiator mass thus forms a central design problem for many space systems.
In most designs the radiator is composed of an array of tubes or tube-fin structures through which flows a coolant. The tubes must be sufficiently massive to minimize micrometeorical penetration; in addition, transport of the coolant over large distances is often required. Radiator mass in such designs often comprises a large fraction of the total system mass.
J. M. Hedgepeth proposed, in "Ultralightweight Structures for Space Power," in Radiation Energy Conversion in Space, Vol. 61 of Progress in Astronautics and Aeronautics, K. W. Billman, ed. (AIAA, New York, 1978), p. 126, the use of a dust radiator to reduce the radiator weight of solar power satellites. A cylindrical column of dust particles is heated radiatively by the working fluid of a thermal engine and is then sent on a 100 m to 10,000 m trajectory to radiate energy into space, and finally is collected, reheated and redirected. While this idea holds promise for sizeable weight reduction, it has significant practical problems, such as the inefficiency of heating the dust by radiation, the difficulty of manipulating a stream of dust, and degradation of the dust itself over time.
The concept of using a stream of liquid droplets as a lightweight radiator for space retains the low-mass advantages of a dust radiator and has the additional advantages of allowing heat transfer by conduction (heat exchanger) and ease of manipulation. To indicate the degree of improvement possible using droplets instead of tube and fin structures to radiate heat, it is noted that radiator performance is characterized primarily by the specific mass, i.e., mass per radiator area. The best tube and fin designs incorporating heat pipes have specific masses of 5-10 kg/m.sup.2. The specific mass of a droplet is simply 1/3.rho.a, where a is the droplet radius. Even with a medium as heavy as liquid tin (.rho.=6.8 g/cm.sup.3), which turns out to be an excellent medium for rejection in space thermal engines, the specific mass is 0.1 kg/m.sup.2 for 0.1 mm dia droplets, a 50-fold improvement over tube-fin radiators.
Collection and transport of the cooled droplets pose a problem. One elementary collection scheme is taught by A. T. Mattick and A. Hertzberg, in "Liquid Droplet Radiators for Heat Rejection in Space, Conference Energy to the 21st Century, Proceedings of the 15th Intersociety Energy Conversion, Engineering Conference, Seattle, Wash. U.S.A. (Aug. 18-22, 1980). This elementary method of collecting the cooled droplets is shown in FIG. 1. The collector is a rotating drum that forms the drop stream into a continuous liquid by centrifugal acceleration. Pumps spaced symmetrically around the periphery of the drum then pressurize the liquid to overcome the centrifugal force and provide back pressure for the main heat exchanger pump. Only modest rotation speeds (a few rpm) are required to collect the liquid for drum diameters above a meter. However, the need for a drum of this size, can prove to be disadvantageous, both in weight and space. Also, insertion of liquid into the rotating collector does present an additional technical complication. Furthermore, a rotary drum collector is not compatible with a sheet of droplets having a long width.